Chemical vapor deposition (CVD) is one of the most widely used deposition processes to coat surfaces. Conventional CVD process is based on thermochemical reactions such as thermal decomposition, chemical reduction, displacement and disproportionation reactions. CVD reaction products find applications in a wide variety of fields; providing hard coatings on cutting tools, protecting surfaces against wear, erosion, corrosion, high temperature oxidation, high temperature diffusion, solid state electronic devices, preparation of fibers made of composite materials, and hermetic coatings.
A number of possible processes have been disclosed up to now for forming deposition films. For instance, for producing films of amorphous silicon deposit, there have been tried the vacuum deposition process, plasma CVD process, CVD process, reactive sputtering process, ion plating process and photo-CVD process, etc. Generally, the plasma CVD process is industrialized and widely used for this purpose.
However, deposition films of amorphous silicon still admit of improvements in overall characteristics including electrical and optical properties, various characteristics of fatigue due to repeated uses or to environmental use conditions. In addition, productivity of depositing silicon-containing films presents problems in product uniformity, reproducibility, and mass-production.
The conventional plasma CVD process, as compared with the conventional CVD process, is a complicated reaction process to deposit amorphous silicon and involves many unknown things in the reaction mechanism. The film formation in the plasma CVD process is affected by a number of parameters e.g., substrate temperature, flow rates and mixing ratio of feed gases, pressure during film formation, high frequency power used, electrode structure, structure of deposition chamber, rate of evacuation, and plasma generation method. These various parameters, combined with one another, sometimes cause an unstable plasma, which exerts marked adverse affects on the deposited film. In addition, the parameters characteristic of the deposition apparatus must be determined according to the given apparatus, so that it is difficult in practice to generalize the production conditions.
The conventional plasma CVD process is regarded at the present time as the best method for the purpose of obtaining amorphous silicon films which have such electrical and optical properties so as to fulfill various application purposes. However, the conventional plasma CVD process requires a high operational temperature and therefore is somewhat limited in applications and substrates.
Thus, there is a need for a process for producing silicon-containing films at low equipment cost and with acceptable film characteristics and uniformity.
Silicon-containing polymeric materials of silicon and hydrogen (hereafter referred to as a--SiH) have emerged as a new class of semiconductors in recent years. Such materials are described, for example, in D. Carlson, U.S. Pat. No. 4,064,521, issued on Dec. 20, 1976. The materials are generated as thin films from the decomposition of silane (SiH.sub.4) in electrical discharges or, less frequently, from the thermal decomposition of silane or higher hydrogen-containing silanes (e.g., Si.sub.2 H.sub.6, Si.sub.3 H.sub.8, etc.) as described in a PCT patent application by A. MacDiarmid and Z. Kiss published as International Publication No. WO 82/03069 dated Sep. 16, 1982.
U.S. Pat. No. 4,459,163, issued Jul. 10, 1984 to MacDiarmid and Kiss, teaches the preparation of amorphous semiconductor material that is suitable for use in a wide variety of devices by the pyrolytic decomposition of one or more gaseous phase "polysemiconductanes," including polysilanes and polygermanes. However, U.S. Pat. No. 4,459,163 is directed toward the formation of semiconductor material only; is based on polysilane precursors; does not utilize a metal catalyst, and does not address the use of halosilanes.
When it is desirable to include additional elemental constituents in silicon-containing films, co-reactants such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) are added to the starting materials. When fluorine is to be incorporated into an silicon-containing film, tetrafluorosilane (SiF.sub.4) is most commonly added to the reactant mixture. This is described for example in U.S. Pat. No. 4,217,374 granted to Ovshinsky and Izu on Aug. 12, 1980.
U.S. Pat. No. 4,374,182, issued Feb. 15, 1983 to Gaul et. al., discloses decomposing halogenated polysilanes at an elevated temperature to prepare silicon. Gaul et al., however, teaches the pyrolysis of polychlorosilanes in the presence of tetrabutylphosphonium chloride catalyst. This distinguishes Gaul et al. from the instant invention directed to silicon-containing coatings produced from halosilanes, disilanes, or mixture of halosilanes in the presence of certain metal catalysts. Gaul et al. does not use metals as catalysts.
U.S. Pat. Nos. 2,606,811, issued on Aug. 12, 1952 to Wagner and 4,079,071, issued on Mar. 14, 1978 to Neale, address the decomposition at elevated temperatures of halogenated disilanes. However, Neale and Wagner are directed toward the reductive hydrogenation of di- and polysilanes for the formation of silanes, and more specifically, monosilanes. Neither Wagner nor Neale teach the vapor phase deposition of silicon-containing films from the thermal decomposition of halosilanes in the presence of certain metal catalysts as taught in the instant invention.
United Kingdom Pat. No. 2,148,328, issued to M. Hirooka, et al., on May 30, 1985, teaches the decomposition of various silanes, including monomeric halosilanes (Si.sub.n X.sub.2n+1, where n=1), cyclic polymeric halosilanes (SiX.sub.2).sub.n, where n is greater than or equal to 3, di- and polysilanes such as Si.sub.n HX.sub.2n+1 and Si.sub.n H.sub.2 X.sub.2n. These materials are decomposed via electric discharge, or photolysis, or high temperature or catalyst and, unlike the instant invention, are mixed with a requisite second stream consisting of a vapor phase material selected from the group consisting of H.sub.2, SiH.sub.4, SiH.sub.3 Br, or SiH.sub.3 I, wherein the second stream has also been decomposed. The obvious disadvantage of such prior art, one which clearly distinguishes it from the instant invention, is the necessity of having two materials to decompose. The Hirooka, et al. patent requires the second stream as the source of hydrogen to facilitate the reduction of the silane to the amorphous silicon. The instant invention, however, can have hydrogen present, if desired, and silicon in the single stream of halosilane to produce the desired silicon-containing coating but, hydrogen is not required. Hirooka et al. does not teach the use of the metal catalysts of the instant invention. Hirooka et al. does disclose the nonessential use of undefined heterogeneous or homogeneous catalysts for the formation of the requisite activated species used as the source of the hydrogen. Hirooka et al. does not teach that the undefined catalysts assist the deposition of the amorphous silicon film, but rather, that the catalysts of Hirooka et al. assist in the formation of the activated species described therein.
Woditsch et al. in the German patent DE No. 3310828-A, issued in 1984, describe the production of silicon for solar cell manufacture by reacting gaseous silane, halosilane or silicon halide with solid aluminum. The reaction temperature is below that of the melting point of aluminum and preferably 500 to 650 degrees C. The preferred silicon compound is silicon tetrachloride and the aluminum is in finely-divided form with a specific surface area of over 0.05 m.sup.2 /g. See also U.S. Pat. No. 4,525,334, issued Jun. 25, 1985, to Woditsh, et al.
H. Schafer, in his book titled "Chemical Transport Reactions," p. 46, (Academic Press, 1964) has described specific examples where metals including iron, cobalt, nickel and copper have been reduced and transported through a vapor phase to form a lustrous mirror on the walls of the reaction vessel. Shafer also reports the deposition of thin silicon foils with metallic luster on a quartz reactor vessel wall when using aluminum in the presence of aluminum chloride (Al.sub.2 Cl.sub.3). (H. Shafer, Z. anorg. Chem., 445, 129-139 (1978)).